Field effect transistor having a multilayer interconnection layer therein with tapered sidewall insulators

ABSTRACT

A field effect transistor and a method of manufacturing thereof are disclosed that is not reduced in the characteristic of withstanding voltage between multilayer interconnection layers even when scaled to a higher integration. This field effect transistor includes side walls 21a formed on both sides of a bit line 15 so that the bottom side end contacts the upper surface of side walls 20a of gate electrodes 4b and 4c. The thickness of an insulating film interposed between gate electrodes 4b and 4c and a base portion 11a forming a low electrode 11 of a capacitor is not reduced. The characteristic of withstanding voltage is not deteriorated between multilayer interconnection layers even when scaled to higher integration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a field effect transistor,and more particularly, to a field effect transistor applied to a DRAM,and a method of manufacturing thereof.

2. Description of the Background Art

In recent years, the demand for semiconductor memory devices is rapidlyincreasing due to the remarkable spread of information processingapparatus such as computers. Semiconductor memory devices havingfunctionally large scale storage capacity and capable of high speedoperation are required. Accordingly, developments in techniques arecarried out regarding high density integration, quick response, and highreliability of semiconductor memory devices.

A DRAM is well known as a semiconductor memory device being capable ofrandom input/output of storage information. A DRAM usually comprises amemory cell array including a storage region for storing a plurality ofstorage information, and a peripheral circuit required for input fromand output to an external source. FIG. 6 is a block diagram showing astructure of a conventional DRAM. Referring to FIG. 6, a DRAM 50comprises a memory cell array 51 for storing data signal of storageinformation, a row-and-column address buffer 52 for receiving externaladdress signals to select memory cells each forming a unit storagecircuit, a row decoder 53 and a column decoder 54 for specifying amemory cell by decoding the address signal, a sense refresh amplifier 55for amplifying and reading out the signal stored in the specified memorycell, a data-in buffer 56 and a data-out buffer 57 for datainput/output, and a clock generator 58 for generating a clock signal.

The memory cell array 51 occupying a large area on the semiconductorchip has a plurality of memory cells arranged in a matrix manner andeach for storing unit storage information. A memory cell is generallyconstituted by one MOS transistor and one capacitor connected thereto.This memory cell is well known as an one-transistor one-capacitor typememory cell. Such memory cells are widely used for large capacity DRAMsbecause of its simple structure contributing to the improvement inhigher integration density of the memory cell array.

FIG. 7 is a sectional view of a memory cell of a conventional DRAM.Referring to FIG. 7, source/drain regions 6 are formed with apredetermined distance therebetween on a P type silicon substrate 1.Gate electrodes 4b and 4c are formed between the pair of source/drainregions 6 with a gate insulating film 5 thereunder. An insulating oxidefilm 200 is formed to cover gate electrodes 4b and 4c, with side walls200a and 200b formed at the side walls thereof. A bit line 150 isconnected to one of source/drain regions 6 formed between gateelectrodes 4b and 4c. An insulating oxide film 210 and side walls 210aand 210b are formed over and at the side walls of bit line 150. A baseportion 11a forming a lower electrode of a capacitor storing charge isconnected to the other of source/drain regions 6. Base portion 11a andgate electrode 4b are insulated by side wall 200a and insulating oxidefilm 200. More specifically, the bottom of side wall 210a formed at theside wall of bit line 150 is located over gate electrode 4b. Insulatingoxide film 200 becomes lower in height at one end of the regionconnecting side wall 210a and insulating oxide film 200. This is due toa manufacturing process that will be described later. In a conventionalmemory cell, insulating oxide film 200 over gate electrode 4b has athick portion and a thin portion with a step at the boundary region.

FIGS. 8A-8D are sectional views of the memory cell of the conventionalDRAM of FIG. 7. The manufacturing process will be described withreference to FIGS. 7-8D. Referring to FIG. 8A, gate electrodes 4b and 4care spaced apart on P type silicon substrate 1 with gate insulating film5 thereunder. Insulating oxide film 200 is formed to cover gateelectrodes 4b and 4c. Source/drain regions 6 are formed, followed byforming side walls 200a and 200b to cover the side walls of gateelectrodes 4b and 4c, and insulating oxide film 200. Referring to FIG.8B, bit line 150 is formed to connect one of source/drain regions 6 on Ptype silicon substrate 1 between gate electrodes 4b and 4c. Side wall200b and insulating oxide film 200 are interposed between gateelectrodes 4b, 4c and bit line 150 to establish withstanding voltage. Aninsulating oxide film 210 is formed on bit line 150. Referring to FIG.8C, an oxide film 30 is formed on the whole surface. Referring to FIG.8D, oxide film 30 is etched anisotropically to form side walls 210a. Informing side walls 210a, a portion of insulating oxide film 200 formedon gate electrodes 4b and 4c is over-etched. This over-etching resultsin a disadvantage that the thickness of a portion of insulating oxidefilm 200 is reduced. When base portion 11a connected to source drainregion 6 is formed to contact side wall 200a, insulating oxide film 200,side wall 210a and insulating oxide film 210 in the above describedstate, insulating oxide film 200 interposed between base portion 11a andgate electrode 4b becomes thinner at the portion where oxide film 200 ongate electrode 4b is over-etched. When a portion of insulating oxidefilm 200 is over-etched to become thinner at that portion due toover-etching in forming side wall 210a, an edge portion is formed at theboundary of the thinner portion and the remaining portion of insulatingoxide film 200.

As mentioned above, a memory cell of a conventional DRAM has insulatingoxide film 200 on gate electrode 4b over-etched in forming side wall210a for establishing withstanding voltage between base line 150 andbase portion 11a constituting the lower electrode of the capacitor. Thiswill result in a configuration where the thickness of insulating oxidefilm 200 becomes thinner in the over-etched portion, with an edgeportion formed at the boundary of the thick and thin film. This posed aproblem that withstanding voltage characteristic between base portion11a and gate electrode 4b is deteriorated. There was another problemthat field concentration occurs at the above-mentioned edge portion.

Conventional semiconductor devices with a multilayer wiring structurescaled to higher densities had a problem that the thickness of theinsulating layer interposed between multilayer interconnection layersbecomes thin to make it difficult to improve withstanding voltage.

SUMMARY OF THE INVENTION

An object of the present invention is to prevent the characteristic ofwithstanding voltage between multilayer interconnection layers fromdeteriorating even when a field effect transistor is scaled to higherdensity.

Another object of the present invention is to improve the characteristicof withstanding voltage between a gate electrode and a lower electrodecf a capacitor in a field effect transistor.

A further object of the present invention is to prevent occurrence oflocal field concentration between a gate electrode and a lower electrodeof a capacitor in a field effect transistor.

A still further object of the present invention is to prevent reductionin film thickness of an insulating film between multilayerinterconnection layers even when scaled to higher densities in a methodof manufacturing a field effect transistor.

Yet another object of the present invention is to prevent an insulatingfilm on a gate electrode from being over-etched in forming side walls ofa conductive layer above a gate electrode in a method of manufacturing afield effect transistor.

According to an aspect of the present invention, a field effecttransistor includes a semiconductor substrate, a pair of impurityregions, a gate electrode, a first insulating layer, a conductive layer,and a second insulating layer. The pair of impurity regions is formed onthe surface of the semiconductor substrate. The gate electrode is formedbetween the pair of impurity regions on the surface of the semiconductorsubstrate with a gate insulating film thereunder. The first insulatingfilm includes a first upper oxide film formed on the gate electrode, anda pair of first side wall insulating films formed on both sides of thegate electrode and the first upper oxide film. The conductive layer isconnected to one of the impurity regions, contacting the side of one ofthe first side wall insulating films, having one end extending over thegate electrode with an insulating film therebetween. The secondinsulating film includes a second upper oxide film formed on theconductive layer, and second side wall insulating films formed on oneside of the conductive layer and the second upper oxide film, with thebottom located on the surface of the other first side wall insulatingfilm, having a portion where the film thickness becomes graduallythicker downwards.

In operation, a second side wall insulating film is formed at one sideof the conductive layer and the second upper oxide film, with the bottomlocated on the surface of the other first side wall insulating film,having a portion where the film thickness becomes gradually thickerdownwards. In the case where a conductive layer is formed over a gateelectrode with a first upper oxide film therebetween, deterioration inwithstanding voltage of the gate electrode and the conductive layer doesnot occur at the junction region of the bottom of a second side wallinsulating film and the first upper oxide film.

According to another aspect of the present invention, a field effecttransistor includes a semiconductor substrate, a pair of impurityregions, a gate electrode, a first insulating layer, a first conductivelayer, a second insulating layer, and a second conductive layer. Thepair of impurity regions is formed on the surface of the semiconductorsubstrate. The gate electrode is formed between the pair of impurityregions on the surface of the semiconductor substrate with a gateinsulating film thereunder. The first insulating layer includes a firstupper oxide film formed on the gate electrode, and a pair of first sidewall insulating films formed at both sides of the gate electrode and thefirst upper oxide film. The first conductive layer is connected to oneof the impurity regions with one side end extending over the gateelectrode with an insulating film therebetween, contacting the side ofone of the first side wall insulating films. The second conductive layerincludes a second upper oxide film formed on the first conductive layer,and second side wall insulating films formed at one side of the firstconductive layer and second upper oxide film, with the bottom located onthe surface of the other of the first side wall insulating films. Thesecond conductive layer connected to the other impurity region is formedto contact the sides of the other of the first side wall insulatingfilms and the second side wall insulating film to be electricallyinsulated from the first conductive layer.

In operation, a second upper oxide film is formed on a first conductivelayer; and a second side wall insulating film is formed at the side ofthe first conductive layer and the second upper oxide film, with thebottom located on the surface of the other of the first side wallinsulating films. A second conductive layer is formed on the otherimpurity region, contacting the side of the other of the first side wallinsulating films and the side of the second side wall insulating film tobe electrically insulated from the first conductive layer. Withstandingvoltage between the gate electrode and the second conductive layer isnot reduced at the junction region of the bottom of the second side wallinsulating film and the first upper oxide film.

According to a further aspect of the present invention, a method ofmanufacturing a field effect transistor comprises the step of forming anelectrode layer on a semiconductor substrate. A first insulating layeris formed including a first upper oxide film covering the electrodelayer and first side wall insulating films covering the sides thereof.Impurity regions are formed adjacent to the first side wall insulatingfilms of the semiconductor substrate. A conductive layer and a secondinsulating film are formed on the semiconductor substrate and the firstinsulating layer, whereby they are patterned to have an end on the firstinsulating film. A third insulating film is formed all over thesemiconductor substrate. The third insulating film is etched to form asecond side wall insulating film contacting one side of the conductivelayer located over the first insulating layer, with the bottom locatedon the surface of the first side wall insulating film.

In operation, a conductive layer and a second insulating layer areformed on the semiconductor substrate and the first insulating layer tobe patterned to a configuration having an end on the first insulatinglayer. A third insulating layer is formed all over the semiconductorsubstrate. The third insulating layer is etched to form a second sidewall insulating film contacting the side of the conductive layerpositioned over the first insulating layer, with the bottom located onthe surface of the first side wall insulating film. This will preventthe first upper oxide film from being removed in forming the second sidewall insulating film, so that withstanding voltage is not reduced.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan layout diagram of a memory cell array of a DRAMaccording to an embodiment of the present invention.

FIG. 1B is a sectional view taken along line X--X of the memory cellarray of the DRAM of FIG. 1A.

FIGS. 2A-2D are sectional views of the memory cell of FIG. 1 fordescribing the manufacturing process.

FIG. 3 is a sectional view of a memory cell array of a DRAM according toanother embodiment of the present invention.

FIG. 4 is a sectional view of a 2-gate FET according to a furtherembodiment of the present invention.

FIG. 5 is a sectional view of a parallel transistor according to a stillfurther embodiment of the present invention.

FIG. 6 is a circuit block diagram showing the structure of aconventional DRAM.

FIG. 7 is a sectional view showing a memory cell of a conventional DRAM.

FIGS. 8A-8D are sectional views of the memory cell of the DRAM of FIG. 7for describing the manufacturing process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1A and 1B, a memory cell array of a DRAM isconstituted by a transfer gate transistor 3 and a capacitor 10. Transfergate transistor 3 comprises one pair of source/drain regions 6 formed onthe surface of a P type silicon substrate 1, and gate electrodes 4b and4c formed on the surface of P type silicon substrate 1 with a gateinsulating film 5 thereunder between one pair of source/drain regions 6.Gate electrodes 4b and 4c are covered with insulating oxide film 20 andside walls 20a and 20b. Side wall 20a is formed thicker than side wall20b. Capacitor 10 is formed of a layered structure of a lower electrode(storage node) 11, a dielectric layer 12, and an upper electrode (cellplate) 13. Lower electrode 11 is constituted by a base portion 11aconnected to source/drain regions 6 formed adjacent to field oxide film2, and a wall 11b formed to expand in a vertical direction along theouter periphery of base portion 11a. Wall 11b of lower electrode 11 iseffective for ensuring constant capacitance when miniaturized, due toboth the inside and the outside thereof implementing the capacitance. Abit line 15 is connected to one of source/drain regions 6 of transfergate transistor 3. Gate electrodes 4d and 4e are formed on a field oxidefilm 2. An insulating oxide film 20 is formed to cover gate electrodes4d and 4e. Gate electrodes 4b, 4c, 4d and 4e are arranged longitudinallywith predetermined distances, as shown in FIG. 1A. The bit lines 15 arearranged laterally with a predetermined distance. An interlayerinsulating film 22 is formed on an upper electrode 13. Interconnectionlayers 18 are, formed over interlayer insulating film 22 at positionscorresponding to gate electrodes 4b, 4c, 4d and 4e, respectively. Aprotect film 23 is formed to cover interconnection layer 18. The featureof the present embodiment lies in the position where side wall 21aformed at the side wall of bit line 15 contacts side wall 20a formed atthe side wall of gate electrodes 4b and 4c. The present embodiment isimplemented so that the bottom of side wall 21a formed at the side ofbit line 15 contacts side wall 20a of gate electrodes 4b and 4c.Insulating oxide film 20 on gate electrodes 4b and 4c is not reduced inthickness, as in conventional semiconductor devices. Therefore,withstanding voltage between base portion 11a and gate electrodes 4b and4c can be improved.

The manufacturing process will be described with reference to FIGS.1-2D. Referring to FIG. 2A, gate electrodes 4b and 4c are formed with apredetermined distance on P type silicon substrate 1 with gateinsulating film 5 thereunder. Insulating oxide film 20 is formed tocover gate electrodes 4b and 4c. Then, source/drain regions 6 areformed. Side walls 20a and 20b are formed to cover the sides of gateelectrodes 4b and 4c, and insulating oxide film 20. Referring to FIG.2B, bit line 15 is formed on source/drain regions 6 between gateelectrodes 4b and 4c. Insulating oxide film 21 is formed on bit line 15.The width W of bit line 15 is formed to be wider than that of aconventional bit line (refer to FIG. 6) in the present embodiment.Referring to FIG. 2C, oxide film 30 is formed all over the surface.Referring to FIG. 2D, oxide film 30 is etched anisotropically to formside walls 21a. Side wall 21a is formed so that the bottom contacts sidewalls 20a of gate electrodes 4b and 4c. Even in the case where baseportion 11a is formed on source/drain regions 6 to contact side walls20a and 21a, as shown in FIG. 1, the characteristic of withstandingvoltage can be improved without reduction of film thickness of theinsulating layer interposed between base portion 11a and gate electrodes4b and 4c. Occurrence of field concentration resulting from insulatingoxide film 20 being over-etched to form an edge is eliminated.

A second embodiment of FIG. 3 differs in capacitor structure from thefirst embodiment of FIG. 1. Capacitor 10 of the second embodiment isconstituted by lower electrode 11 formed of base portion 11a connectedto source/drain regions 6 and wall 11b formed in a region sandwiched byinsulating films 26, dielectric layer 12 formed to cover low electrode11, and upper electrode 13 formed on dielectric layer 12. An oxide film24 is formed on insulating oxide film 21 above bit line 15, and fieldoxide film 2. An insulating layer 25 is formed on oxide film 24. Bitline 15 is formed to have a large width, similarly as in the firstembodiment of FIG. 1, with side walls 21a formed at the sides. Thebottom of side wall 21a is formed to contact the surface of side wall20a of gate electrodes 4b and 4c. This structure will give an effectsimilar to that of the first embodiment of FIG. 1. In other words, thecharacteristic of withstanding voltage can be improved between baseportion 11a forming lower electrode 11 of the capacitor and gateelectrodes 4b and 4c.

Referring to FIG. 4, a 2-gate FET has a pair of source/drain regions 6formed on P type silicon substrate 1. An interconnection layer 48 isformed above source/drain regions 6 with a contact layer 47therebetween. Two gate electrodes 44b and 44c are formed between onepair of source/drain regions 6. Gate electrode 44c extends over gateelectrode 44b with insulating oxide film 42 and side wall 42btherebetween. Insulating oxide film 43 is formed on gate electrode 44c.Side walls 43a and 43b are formed at the sides thereof. Interlayerinsulating film 45 is formed to cover the whole surface. The feature ofthe present invention lies in that one side of the bottom of side wall43a of gate electrode 44c contacts smoothly with the upper surface ofside wall 42a of gate electrode 44b. Coverage of interlayer insulatingfilm 45 at the contact region of side wall 42a and side wall 43a is notdeteriorated, in forming interlayer insulating film 45 over side walls42 a and 43a. Reduction in reliability of the two-gate transistor can beprevented effectively.

Referring to FIG. 5, a parallel transistor has source/drain regions 6with a predetermined distance formed on P type silicon substrate 1. Gateelectrodes 4b and 4c are formed between source/drain regions 6 with gateinsulating film 5 thereunder. An interconnection layer 55 is formed onthe source/drain region 6 sandwiched between gate electrodes 4b and 4c.Insulating oxide film 21 and side walls 21a and 21b are formed overinterconnection layer 55. Interconnection layer 53 is formed oversource/drain region 6 with a contact layer 52 therebetween. Substrate 1is covered with interlayer insulating film 51 except where contact layer52 is formed. The feature of the fourth embodiment lies in that thecontact of one end of the bottom of the side wall 21a formed on bothsides of interconnection layer 55 and the insulating layer covering gateelectrodes 4b and 4c is located on the surface of side wall 20a. Thisgives an effect similar to that of the third embodiment. In other words,coverage of interlayer insulating film 51 between the junction region ofside walls 20a and 21a is not deteriorated in forming interlayerinsulating film 51 on side walls 20a and 21a. Therefore, reduction inreliability of the parallel transistor can be prevented effectively.

As described in the above embodiments, the side wall structure of thepresent invention can be applied to various semiconductor devices.Although the manufacturing process of the first embodiment of FIG. 1 wasshown in FIGS. 2A-2D, the manufacturing process of the present inventionis not limited to this. For example, a similar configuration can beobtained by bit line 15 having a width similar to that of a conventionalone and by increasing the thickness of oxide film 30 in forming sidewalls 21a at the sides of bit line 15 to increase the thickness of sidewalls 21a. A side wall 21a similar to that shown in FIG. 1 can be formedby employing both the methods of increasing the width of bit line 15 andthe thickness of oxide film 30 at the time of forming side walls 21a.

In accordance with a concept of a field effect transistor of the presentinvention, a first insulating layer is formed having a first upper oxidefilm on a gate electrode, and a pair of first side wall insulating filmson both sides of the gate electrode and the first upper oxide film. Aconductive layer is formed at one of the impurity regions formed on thesurface of the semiconductor substrate, contacting one side of one ofthe first side wall insulating films, with one end extending over thegate electrode with an insulating film therebetween. A second insulatinglayer is formed having a second upper oxide film on the conductive layerand a second side wall insulating film with the bottom end of theconductive layer and second upper oxide film located on the surface ofthe other of the first side wall insulating films. In the case aconductive layer is formed on the electrode layer with a first upperoxide film therebetween, withstanding voltage of the electrode layer andthe conductive layer is not reduced in the junction region of the bottomof the second side wall insulating film and the first upper oxide film.Therefore, a field effect transistor is provided that allows improvementof withstanding voltage between multilayer interconnection layers evenwhen scaled to higher integration.

In accordance with another aspect of a field effect transistor of thepresent invention, a first insulating film is formed having a firstupper oxide film on a gate electrode, and a pair of first side wallinsulating films on both sides of the gate electrode and the first upperoxide film. A first conductive layer is formed at one of the pair ofimpurity regions formed on the surface of the semiconductor substrate,connected to the side of one of the first side wall insulating films,with one end extending over the gate electrode with an insulating filmtherebetween. A second insulating layer is formed having a second upperoxide film on the first conductive layer and a second side wallinsulating film with the bottom of one side of the first conductivelayer and the second upper oxide film located on the surface of theother of the first side wall insulating films. A second conductive layeris formed at the other of the impurity regions, contacting the side ofthe other of the first side wall insulating films and the side of secondside wall insulating film to be electrically insulated from the firstconductive layer. This gives the advantage that withstanding voltage ofthe electrode layer and the conductive layer is not reduced in thejunction region of the bottom end of the second side wall insulatingfilm and the first upper oxide film. Therefore, a field effecttransistor is provided that allows the improvement of withstandingvoltage between multilayer interconnection layers even when scaled to ahigher density.

In accordance with a further concept of the present invention, a methodof manufacturing a field effect transistor comprises the step of formingan electrode layer on a semiconductor substrate. A first insulating filmis formed constituted by a first upper oxide film covering the electrodelayer and a first side wall insulating film covering the side. Animpurity region is formed at a region adjacent to the first side wallinsulating film of the semiconductor device. A conductive layer and asecond insulating layer is formed on the semiconductor substrate and thefirst insulating layer to be patterned to have the side over the firstinsulating layer. A third insulating film is formed all over thesemiconductor substrate. The third insulating layer is etched to form asecond side wall insulating film contacting the side of the conductivelayer located over the first insulating layer, with the bottom sidethereof located on the surface of the first side wall insulating film.This gives the advantage that the first upper oxide film is not removedin forming a second side wall insulating film so that withstandingvoltage is not reduced. Therefore, a method of manufacturing a fieldeffect transistor is provided that allows improvement of withstandingvoltage between multilayer interconnection layers even when integratedto higher density.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A field effect transistor comprising:asemiconductor substrate (1), a pair of impurity regions (6) formed onthe surface of said semiconductor substrate, a gate electrode (4c)located between said pair of impurity regions, and formed on the surfaceof said semiconductor substrate with a gate insulating film thereunder,a first insulating layer (20, 20a, 20b) having a first upper oxide film(20) formed on said gate electrode, and a pair of first side wallinsulating films (20a, 20b) formed on both sides of said gate electrodeand said first upper oxide film, a conductive layer (15) connected toone of said impurity regions, contacting the side of one of said firstside wall insulating films, with one side end extending over said gateelectrode with said first upper oxide film therebetween, and a secondinsulating layer (21, 21a) having a second upper oxide film (21) formedon said conductive layer, and side of said conductive layer and saidsecond upper oxide film, with the bottom side end located on the surfaceof the other of said first side wall insulating films, having a portionwhere film thickness becomes gradually thicker downwards.
 2. The fieldeffect transistor according to claim 1, wherein said second side wallinsulating film (21a) is formed so that the bottom side end is locatedoutside the side wall of said gate electrode (4b, 4c).
 3. The fieldeffect transistor according to claim 1, wherein said other of first sidewall insulating films (20a) is formed thicker than said one of saidfirst side wall insulating films (20b).
 4. The field effect transistoraccording to claim 2, wherein said other of said first side wallinsulating films (20a) is formed thicker than said one of said firstside wall insulating films (20b).
 5. A field effect transistorcomprising:a semiconductor substrate (1), a pair of impurity regions (5)formed on the surface of said semiconductor substrate, a gate electrode(4c) located between said pair of impurity regions, and formed on thesurface of said semiconductor substrate with a gate insulating filmthereunder, a first insulating layer (20a, 20b) having a first upperoxide film (20) formed on, said gate electrode, and a pair of first sidewall insulating films formed on both sides of said gate electrode andsaid first upper oxide film, a first conductive layer (15) connected tosaid one of impurity regions, contacting the side of said one of saidfirst side wall insulating film, with one side end extending over saidgate electrode with said first upper oxide film therebetween, a secondinsulating layer (21, 21a) having a second upper oxide film (21) formedon said first conductive layer, and a second side wall insulating layer(21a) formed on one side of said first conductive layer and said secondupper oxide film, with the bottom side end located on the surface ofsaid other of said first side wall insulating film, and a secondconductive layer (11a) connected to said other of impurity regions,contacting the side of said other of said first side wall insulatingfilms and the side of said second side wall insulating film to beelectrically insulated from said first conductive layer.
 6. The fieldeffect transistor according to claim 5, wherein said second side wallinsulating film (20a) comprises a portion where the film thicknessthereof gradually increases downwards.
 7. The field effect transistoraccording to claim 6, wherein said second side wall insulating film(21a) is formed so that the bottom side end is positioned outside theside wall of said gate electrode (4b, 4c).
 8. The field effecttransistor according to claim 6, wherein said other of said first sidewall insulating films (20a) is formed thicker than said one of saidfirst side wall insulating films (20b).
 9. The field effect transistoraccording to claim 7, wherein said other of said first side wallinsulating films (20a) is formed thicker than said one of said firstside wall insulating films (20b).
 10. The field effect transistoraccording to claim 5, whereinsaid first conductive layer comprises a bitline (15), and said second conductive layer comprises a low electrode(11) of a capacitor.
 11. The field effect transistor according to claim10, wherein said low electrode (11) of the capacitor is formed so thatthe end extends in a direction perpendicular to said semiconductorsubstrate.
 12. A field effect transistor comprising:a semiconductorsubstrate (1), a pair of impurity regions (6) formed on the surface ofsaid semiconductor substrate, a first gate electrode (44b) adjacent tosaid one of impurity regions, formed with a predetermined width on thesurface of said semiconductor substrate with a first gate oxide filmthereunder, a first insulating layer having a first upper oxide film(42) formed on said first gate electrode, and a pair of first side wallinsulating films (42a, 42b) formed on both sides of said first gateelectrode and said first upper oxide film, a second gate electrode (44c)adjacent to said other of impurity regions, contacting the side of saidone of first side wall insulating films, with a predetermined width onthe surface of said semiconductor substrate with a second gate oxidefilm thereunder, wherein one side end extends over said first gateelectrode with said first upper oxide film therebetween, and a secondinsulating layer (21, 21a) having a second upper oxide film (21) formedon said second gate electrode, and a second side wall insulating film(21a) formed on one side of said second gate electrode and said secondupper oxide film, with the bottom side end located on the surface ofsaid other of first side wall insulating films, having a portion wherefilm thickness becomes gradually thicker downwards.
 13. The field effecttransistor according to claim 12, wherein said field effect transistorcomprises a two-gate transistor.
 14. The field effect transistoraccording to claim 1, wherein said field effect transistor comprises aparallel transistor.
 15. A field effect transistor comprising:asemiconductor substrate, a transfer gate transistor having a pair ofsource/drain regions formed on the surface of said semiconductorsubstrate, and a gate electrode formed between said pair of source/drainregions with a gate oxide film thereunder, a bit line connected to saidone of impurity regions, formed at the side wall and over said gateelectrode with a first insulating film therebetween, a second insulatingfilm formed on said bit line, and a low electrode of a capacitorconnected to said other of impurity regions, formed at the side walls ofsaid gate electrode and said bitline with said fist insulating film andsaid second insulating film therebetween, respectively, wherein saidside wall of said second insulating film comprises a portion where thefilm thickness thereof gradually increases downwards, and wherein saidfirst insulating film and said second insulating film are formed to joinsmoothly at the junction region thereof.